Forces and Motion

Head on collision between trolleys with magnets attached

Practical Activity for 14-16 PRACTICAL PHYISCS


"Invisible" collisions: collisions when objects do not touch.

Apparatus and Materials

  • Dynamics trolleys, 2
  • Horseshoe magnets, Alnico, 2
  • Sellotape
  • 0-Gauge railway track with trucks (OPTIONAL)

Health & Safety and Technical Notes

Read our standard health & safety guidance

Fix the strong magnets onto the ends of the dynamics trolleys. The magnets are repelling each other. Use Sellotape to secure them satisfactorily.

Alternatively, you could use trucks on a gauge-0 railway track or gliders on an air track.


Show collisions between these trolleys when they are pushed gently towards each other.

Teaching Notes

  • The collision is silent as the trolleys/trucks do not physically collide. When the magnets on the trolleys/trucks come in range their magnetic fields interact and the trolleys/trucks are repelled. Momentum is transferred to the magnetic field.
  • You could say: "This demonstration is important as a reminder that in collisions we do not need to have contact. Is there really any contact at the atomic level? In fact all that happens is that forces rise steeply to big repulsions as the atoms move closer. The repulsions grow so large that, even though they have a very short time to act during the collision, they are able to bring the colliding bodies to a stop and push them apart again. There is always a distance of closest approach. This grows smaller and smaller as the collisions become more violent when colliding bodies are projected towards each other at higher and higher speeds."
  • The attractions between atoms or molecules are short range electrical forces extending out a few molecular diameters at the most. See the experiment Soap film; the tension remains constant for thin films:

    Soap film

    There must also be repulsions or else solids and liquids would collapse under the attraction. These are very short range, a small fraction of an atomic diameter.
  • We think of the repulsive forces as not appearing until one atom goes ‘smack’ against another in a collision. There is no real ‘smack’, only a very sudden slowing and stopping and starting away again of an atom in the strong force field of another atom’s repulsion.
  • Considering only one atom the equilibrium position in which it should settle down is:
    • At point X where the two forces just balance (where the resultant force is zero)
    • Where the potential energy is minimum at point Z.
  • In practice the atom does not settle down but remains in oscillation about the equilibrium point.
appears in the relation p=mv F=dp/dt λ=h/p ΔxΔp>ℏ/2
has the special case Angular Momentum
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